1. Field of Invention
Polyurethane elastomers are used as sole materials in industrial safety shoes. Compared with rubber, these products offer better wear resistance. However, at relatively low temperatures they begin to soften or melt. For this reason, the use of polyurethane soles has been restricted to normal industrial safety shoes.
The high temperature dimensional stability of polyurethane elastomers could be improved by using aromatic diamines instead of diols as the chain extenders. Polyurethane elastomer systems containing diamine, however, can only be processed in high-pressure RIM equipment. Polyurethane based shoe soles are currently produced almost exclusively using the low-pressure process.
2. Description of Prior Art
The preparation of dense or cellular polyurethane elastomers by reacting organic polyisocyanates with organic polyhydroxyl compounds in the presence of catalysts and optionally chain extenders and/or cross-linking agents as well as blowing agents, auxiliaries, and/or additives is known in the art. lf the hydroxyl group-containing polyesters, polyethers, polyester amides, etc., and organic polyisocyanates are correctly selected and when chain extenders such as glycols, trifunctional alcohols, and/or diamines are used, both elastic and rigid, optionally cellular polyurethane elastomers as well as all intermediate modifications can be prepared by known methods.
The preparation of polyurethane elastomers, their mechanical properties, and uses is described, for example, in the monographs Polyurethanes, Parts I and II, Vol. XVI, of High Polymers, by I. H. Saunders and K. C. Frisch (New York: Interscience Publishers, 1962 and 1964) and Polyurethane, Vol. VII of Kunststoff-Handbuch, Ed. R. Vieweg and A. Hochtlen, G. Oertel (Munich: Carl Hanser Verlag, 1966, 1983).
According to DE No. A-831 604 (U.S. Pat. No. 2,778,810), isocyanate group-containing prepolymers are first prepared from the hydroxyl group-containing, relatively high molecular weight compounds and organic polyisocyanates. These prepolymers are then reacted in a second step with the chain extenders, e.g., diols or diamines, to form high molecular weight elastomers.
Diamines cannot generally be used in a one-shot process. However, the development of high-pressure RIM techniques has permitted the preparation of optionally cellular, elastic molded articles possessing a dense surface of polyurethane-polyurea elastomers by reacting a one-shot system, which essentially comprised organic polyisocyanates, polyols, aromatic diamines and/or polyamines whose amino groups were stearically hindered by ortho-positioned alkyl substituents, and a strong catalyst for the reaction between the hydroxyl and isocyanate groups, in a closed mold. Such processes are described, for example, in DE No. A-27-22-951 (U.S. patent No. 4,218,543) and EP Nos. A-26,915 and 69,286.
In addition, EP No. A-19,213 describes the use of low molecular weight aromatic diamines in a special process used to avoid high gas pressures in the preparation of polyurethane elastomers. The added diamines, in combination with glycols and relatively large amounts of water as cross-linking agents and to form carbon dioxide as a blowing agent, produce polyurethane elastomers having a density of from 0.45 to 0.8 g/cm.sup.3 and improved physical properties at elevated temperatures.